How can I tell if my motorhome battery is defective?


The question is repeatedly asked, and in the vastness of the Internet, I have not been able to find a sensible answer. So here is my attempt to provide answers to the following questions:

  • My battery isn’t taking any more charge – is it defective?
  • How can I check whether the auxiliary battery in the camper is defective?
  • The battery only lasts 3 hours; then the light goes out – defective?
  • Can I have a battery tested at ATU?

When is a lead acid battery defective?A lead-acid battery is generally considered defective when it can no longer supply electricity, and the remaining capacity has dropped to 0. In normal operation, a defective battery is usually only recognized when it is too late, more power than usual is needed, or the battery is discharged a little deeper. For example, when the solar system no longer has as much power in autumn, the nights are getting longer, and more electricity is needed for lighting and heating. Autumn is when most people realize that a motorhome deep cycle battery might not be good anymore.

AGM vs lead acid:AGM batteries have a fiberglass mat that absorbs the acid, while lead-acid batteries have a liquid acid solution. AGM batteries do not require maintenance during charging, while lead-acid batteries need to be maintained to prevent overcharging. AGM batteries have a longer cycle life than lead-acid batteries. AGM batteries have a lower self-discharge rate compared to lead-acid batteries. AGM batteries are generally lighter in weight than lead-acid batteries. Lead-acid batteries have higher CCA ratings than AGM batteries. Lead-acid batteries are generally more cost-effective than AGM batteries. AGM batteries are more expensive than lead-acid batteries but offer longer cycle life and better performance in low temperatures. Lead-acid batteries, on the other hand, provide more power for starting engines and have a lower self-discharge rate.

There are two possible causes of a battery failure

  1. Cell closure

One or more cells in 12 volt battery have a short circuit and can no longer be charged. It is easy to test whether a lead battery has a short circuit. The battery must maintain a voltage of 12.8-13V when fully charged. So if the battery voltage is below 12V after 1 hour, there is a high probability of a cell short circuit. In most cases, a short circuit can also be recognized because the charger does not reach the final charging voltage, the battery heats up considerably, and the smell of rotten eggs (the supply battery is boiling).

With a slight cell closure, however, it can happen that the cell still works but discharges faster. A fully charged battery must still have a voltage of 12.8V even after days (starter batteries 12.5-12.6V).

  1. Age-related failure of the mobile home battery

>From day one, a lead-acid battery loses a little capacity with each discharge, just like a car tire loses a little profile with every kilometer. However, as a normal user, you only notice this wear and tear when the battery’s capacity has dropped so much that it is no longer sufficient for normal operation or in autumn or winter when more capacity is required than in summer.

Check defective motorhome battery

I keep getting called to mobile homes with allegedly defective batteries, and the battery is not always really defective. So I would now like to explain how you can find out for yourself whether a battery is really broken or whether the charger is no longer working properly – or not at all. The basic requirement is a reasonable multimeter with a digital voltage display. And it knows how to use it.

Make sure the battery is fully charged

There can be two reasons if the battery is only enough for 2 hours of television in the evening. Either it was not fully charged, or it was defective. It is, therefore, very important that the battery is charged to 100%. I know it’s time-consuming because you must stay with the camper, but there’s no other way.

Everyone with a battery computer has it much easier because they can read all the values on the monitor. Motorhomes with a current indicator (ammeter) installed on the control panel are also advantageous. Without this current display, the result will be somewhat less precise.

Measuring battery voltage before turning on the battery charger in the RV makes it easier to see if it is working.

Charge the battery: To do this, plug the mobile home into the 230V shore power. Now the voltage of the battery must increase. If it doesn’t, there is a problem with the charger.

After the first big jump of a few tenths of a volt after the charger was switched on, the voltage will slowly rise. Depending on the battery’s charge state, it may take some time before the absorption voltage is reached.

If the battery was already charged beforehand, the voltage drops very quickly, too, e.g., B. 14.3 volts (the actual value can be found in the operating instructions for the charger). The emptier the battery was and the worse its condition, the longer it took for the charger to reach this voltage. Therefore, it may be that you need some patience. It would help if you observed how the tension rises. Because when the absorption charge is complete, the charger drops the voltage to 13.6-13.8V, and you will never know if the charger is working properly. So if you look at the battery again the next day, you will see this voltage there. But I don’t know whether the charger reached the important absorption voltage above 14V. So stick with it, or at least check values such as voltage and current every 15 minutes (if possible). It is best to measure directly on the battery with the multimeter, or you should at least compare once whether the voltage measured directly on the battery corresponds to the value on the display in the mobile home.

When the absorption voltage has been reached and has lasted for a certain amount of time, the battery should be full. Or at least pretty full. The ancillary battery must not be warm or even hot!

Now we have a defined starting point to reliably test the battery. The charger is working correctly; the RV battery is fully charged. Normally, it must now be able to supply electricity for a few hours.

Battery test

Discharge test with battery computer

For example, an 80Ah gel battery must be able to deliver a current of 4A for 20 hours. After that, it is completely discharged. After 10 hours, it is logically 50% discharged, and this value can be tested. Such a discharge test is quite simple with a battery computer, which can also display the consumed ampere hours (Ah). Simply switch on as many consumers as light and TV in the mobile home until around 1/20 of the battery capacity is present as a discharge current and let it run. Now please read the battery voltage every 15 minutes. When it reaches 11.8V, switch everything off and let the battery rest for 30 minutes.

EXAMPLE: 80Ah battery: 20 = 4 amps >> Run a 4 amp device (48 watts: 12V = 4 amps) such as a fan, lights, TV, etc.

The battery voltage should then be around 12V. Now you can read off the battery computer how many ampere hours have been consumed from the battery. This value times two corresponds to the remaining capacity.

Discharge test without battery computer:

It can also be done without a battery computer; just not that easy.

Please remove the fully charged battery for testing. You must have a defined load. For example, a 55-watt light bulb from the headlight. ATTENTION, it gets very hot, and you must not touch anything! Fire hazard! Therefore also, remove the battery and place it on a non-flammable surface. With an 80Ah battery, the discharge current should be around 4A, as mentioned above. A 55-watt H4 halogen bulb requires about 4.4A at 12.5V.

Now connect the bulb directly to the battery and note the start time. Now you should check the battery voltage every 15 minutes. When it reaches 11.8V, the lamp is disconnected. The number of hours x 4.4A now gives the battery’s remaining capacity. A defective battery will usually collapse after just a few hours. A clear indication that you can buy a new battery.

If the camper battery is defective, the attempt to discharge will probably be done quickly because the voltage will collapse quickly! It is normal for the voltage to rise above 12V again after the collapse.

Revive defective supply battery?

Battery pulser is the keyword that people keep asking me about. It’s like this: Batteries that often cannot be fully charged for a long time and are subject to a certain degree of sulphation can be revived with a battery pulser. A battery pulser will not be able to bring a worn-out battery back to life, nor will batteries with a dead cell.

Battery test in the workshop – quick test

Very important: Quick battery test devices can only test a battery for starting ability (high resistance or not). Worn-out batteries with only a low capacity are not recognized. The devices only check the starting ability of a battery (=to start a car) and not the capacity. I have already had customers for whom ATU has certified a 100% good battery, and they were still sitting in the dark camper two hours after sunset with an empty battery. The battery bank with 400Ah only had 15Ah remaining capacity. With this capacity, you could easily start a car but not keep the power supply for a mobile home for long.

Consultation with my battery suppliers showed that batteries are tested in the factory as described here. Of course, there are measuring devices in the five-digit price segment that can record and test a few more values. But my discharging attempt described above is quite sufficient for a simple test.



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